Photoelectrodes with Polydopamine Thin Films Incorporating a Bacterial Photoenzyme

A fabrication strategy of photoactive biohybrid electrodes based on the immobilization of the bacterial reaction center (RC) onto indium tin oxide (ITO) is proposed. The RC is an integral photoenzyme that converts photons into stable charge-separated states with a quantum yield close to one. The photogenerated electron-hole pair can be eventually exploited, with suitable redox mediators, to produce photocurrents. To this purpose, RC must be effectively anchored on the electrode surface and simple strategies for its stable immobilization ensuring prolonged enzyme photoactivity are strongly desired. In this work, polydopamine (PDA), a polymer reminiscent of the natural melanin, is used to anchor the RC on the electrode surface. PDA is easily prepared in situ by spontaneous polymerization of dopamine in slightly alkaline aerated buffered RC solution. This reaction, carried out in the presence of an ITO substrate dipped into the solution, directly leads to a stable RC-PDA/ITO photoelectrode with 20 nm film thickness and 50% of fully functional RC occupancy. Photocurrents densities recorded using this photoelectrode are comparable to those obtained with far more sophisticated immobilization techniques. The RC-PDA films are fully characterized by visible-near-infrared absorption spectroscopy, ellipsometry, atomic force, and scanning electron microscopies.